Process for annealing stainless steel



PROCESS FOR ANNEALI'N'G STAINLESS stain- Charles D. Branson amt .n'sl ii A. ='Sniitli nsert-ins, Temn, assigno'i's td'Robertshaw-Fulton Coiitr'tilbfCo Q- pany, Knxville,':Telin ficorporgtio r of Delawa'ie' This. invention relates to the annealing of s less steel in an atmosphere of dissociatedammonia Stabilized stainless steel is stainless steeltowhich has been added columbium mir'aniumin amoum'sappro mating 6 to 10 times the carbon-content'ofqth'estaitiless Columbium or titanium are added fto stai steels to prevent migration of carbon to 'graln bound at high temperatures and such stabilized star are used for high temperature a'pplieat steel stabilized with' columbium has' b can Iron and Steel Institute Type No. 47"an'd st steel stabilized withtitaniunrfhas been "giifen fim Iron and Steel Institute '1ypeNo. 321: fli'eieihaftt stainless steels will be referred to -by these *typeminibers. s

Heretofore it has been propqsedto anneal stainless steels, including stabilized stainless" 'steels, in, atmospheres of hydrogen and in atmospheres of dissociated ammonia. The use of hydrogen atmospheres" in the annealing of these stainless steels is expensivevparticularly where the annealing is performed in continuousgiopfii tioti since the atmosphere within the furnace is dissipated out of both ends of the furnace. Further, in, the case-.of hydrogen, the oxygen content thereofmust be carefully remoyed. Dissociated ammonia providesa "relatively inexpensive atmosphere and ammonia may be dissociated with conimercially available-equipment., v I

When ammonia is dissociated'itis broken down=info.':its components of N2 and H2. An atmosphere of N2 and H2 presents little difliculty in the annealing of other than stabilized stainless steels, Inj'the casegpf stabilized stainless steels, however, the nitrogen in this atmosphere .Will rea ct with the columbium or titanium in the steel and suchjatre action will free the carbon of the stainless steel for migration to grain boundaries'where the'ca'rbon willcombine with chromium and will reduce the chromium available to prevent corrosion. Under such circumstances the annealing of stabilized stainless 'ste'el'in 'atattriosphere of dissociated ammonia is very damaging to the stainless steel and the stainless steel is thereafter unstabilized for practical purposes subject to corrosion: "and to rapid decomposition with certain corroding agents after certain heat treatments.

We have found that the absorption of nitrogen by a stabilized stainless steel' can be prevented and the effect of the nitrogen content of an atmosphere of dissociated ammonia'can be neutralized if a small amount of an oxidizing agent is added to the atmosphere of dissociated ammonia. This oxidizing agent may be air or water or any other well known oxidizing agent. We prefer to use water for this purpose and our process will be described in more detail below using water as the oxidizing agent. It is to be recognized, however, that our invention is not limited to the use of water for this purpose and other known oxidizing agents come within the scope thereof.

It is accordingly the object of the present invention to provide a process for the annealing of stabilizedstainless steels in an atmosphere containing nitrogen as in an atmosphere of dissociated ammonia in which the harmful effects of the nitrogen content of the atmosphere on the stabilized stainless steel are neutralized by the inclusion of a controlled amount of an oxidizing agent in the atmosphere.

Stainless steels types 347 and 321 can be annealed in temperatures ranging from 1750 F. to 2050 F. in

. tainless steel and more particularly to the annealing of stabilized stain- 2,703,298 C6 Eatentedltls J 55 .i' 15, thin atmospheres of dissociateda'riimonia. Such atmo'spheres maybe readily provided using commercially available equipment in which the ammonia'is passedthrougha dissociation chamber in the presence of a metal catalyst such as mckel or cast iron at temperatures approaching 1750 F. Thereafter the dissociated ammonia is led to the-annealing chamber. We prefer to add the oxidizing agent to the atmosphere of dissociated ammonia by bubbling a small per cent of the -atmosphere,say 3%, through a water column before it is admitted to'the furnace; The dissociated ammonia bubbled through the water column picks up water which will act as the oxidizing'agent in the furnace. i The amount of the'atmosphere'bubbled through the water column must be controlled so' that the dew point of the atmosphere within the furnace is maintainedat a predetermined point for a given temperature it being recognized that the higher the temperature in the furnace, the more moisture must be provided in theatm'osphere of dissociated ammonia. We have also found that furnace design and other considerations may alter the absolute minimum quantity of moisture that must be added to' the atmosphere for satisfactory protection of the steel to be annealed. The following ranges of conditions produce satisfactory protection of the stainless steel Within the scope of our invention: 1 r

Dew point of r When stabilized stainless steel is introduced into an ,fannealing furnace in the presence of an atmosphere-of dissociated ammonia containing water as the oxidizing agent an oxide coating is flashed upon the stainless steel. At the starting temperature for annealing of approximately 1750 F. the hydrogen of the atmosphere is notstrongly reducing in character but the water vapor is highly oxidizing in character. As the temperatures rise in the annealing furnace these characteristics reverse. The moisture in the atmosphere must be suflicient to provide'oxidation at maximum anuealingtemperatures so that. there will always be'sufi'icient oxide coating on the stabilized stainless steel to inhibit it against nitrogen absorption. As the stabilized stainless steel cools down after annealing some oxidation thereof will take place but we have found that this oxidation does not tarnish the stainless steel sufiiciently to require polishing and'the stainless steel annealed by our process is usably bright.- If too much of the oxidizing agent is present in the atmosphere it is apparent that upon cooling excessive scaling of the stainlesssteel will take place. It is therefore essential that the amount of the oxidizing agent employed'be only suflicient to neutralize the nitrogen present at the maximum annealing temperatures.

We have conducted tests upon stainless steeltype 347 which was developed for the purpose of resisting loss of corrosion resistance when heated for long periods of time in the annealing temperature range. Stainless steel 347 absorbs and reacts with nitrogen when heated in this range. Absorption of nitrogen by this steel has been demonstrated in quantitative chemical analysis of the steel. The nitrogen content of a specimen of steel type 347 increased from .078% to .47% after annealing at 1950 F. for one hour. This is an increase in nitrogen by a factor of about 6. A sample of this stainless steel was bright annealed at 2050 F. in a dry atmosphere of dissociated ammonia for 15 minutes and a duplicate sample was annealed at 2050 F. in an atmosphere of dissociated ammonia to which had been added a small amount of water vapor. The samples were then subjected to corrosion in a solution of 10% nitric and 2.3% hydrofluoric acids at F. The corrosion rate of the specimen which had been bright annealed in the dry atmosphere had an 18% greater average penetration rate than the specimen which was annealed with water vapor protection. When similar specimens were heated to 1200 F. for two hours and tested in the same solution the loss of stability of the nitrided steel became obvious. The nitrided specimen completely dissolved at the end of the first hour of corrosion while the specimen annealed with water vapor protection was still intact at the end of four test periods of one hour each.

The loss of stability of nitrided type 347 steel when heated at 1200 F. for two hours with a decrease in corrosion resistance to the nitric-hydrofluoric acid solutions was accompanied by a characteristic change in microstructure. This steel in its normal annealed condition contains no clearly defined grain boundary constitutent when the specimen is polished and electrically etched in 10% oxalic acid. When the nitrided steel is sensitized and electrically etched in 10% oxalic acid a pronounced continuous grain boundary constituent is developed. Subsequent corrosion is rapid and produces a rough granular surface on the steel.

We have determined that type 347 stainless steel specimens such as corrugated expandible and contractible stainless steel vessels or bellows may be stress relieved and stabilized at 1650 F. for four hours. in a dissociated ammonia atmosphere with a small amount of moisture added for increasing corrosion resistance with less oxidation and surface damage than is encountered if the parts are given the same treatment in air. The dissociated am monia atmosphere was humidified to a dew point of approximately 24 F. The following results were obtained.

Lot No. l stabilized (1650 F. for 4 hours) in air.

Lot No. 2 stabilized (1650 F. for 4 hours) in protective atmosphere of dissociated ammonia.

1 One bellows had not. failed at 5,316.180 cycles.

The bellows were identical prior to this treatment.

After the stabilizing heat treatment. the bellows were cleaned in the standard nitric-hydrofluoric acid pickle for stainless steel prior to the fatigue test. The bellows which had been stabilized in the air had markedly greater surface scale. the protective atmosphere of dissociated ammonia had a smooth dark stained surface rather than a scaled surface. After the bellows were pickled they were examined under a microscope. The surface of the bellows that had been air stabilized was much rougher than the one which was stabilized in the dissociated ammonia atmosphere and the greater metal loss was evidenced by the lower spring rate of the bellows.

We have found no temperature limit to the successful application of our process. We have conducted annealing tests from 1750 F. through 2050 F. This tem- The bellows which had been stabilized in perature range is the range generally recommended for the annealing of stainless steels.

It will now be apparent that by the present invention we have provided a novel process for the annealing of stabilized stainless steel in an atmosphere containing nitrogen as in an atmosphere of dissociated ammonia in which reaction of the nitrogen in the atmosphere upon the stainless steel is prevented by inclusion of an oxidizing agent in the atmosphere. It is also apparent that while we prefer to use water as the oxidizing agent in our novel process that other well known oxidizing agents are equally suited for use therein and that our invention is not limited to the use of water for this purpose.

Changes in or modifications to the described embodiment of our novel process may now be apparent to those skilled in the art without departing from our inventive concept. Reference should therefore be had to the appended claims to determine the scope of our invention.

What is claimed is:

1. In a process for annealing columbium or titanium stabilized stainless steel in an atmosphere of dissociated ammonia, the step of adding water vapor to the annealing atmosphere in amounts sufficient to form and maintain an oxide coating on the steel throughout the anneal by bubbling a portion of the dissociated ammonia through water before admitting it to the presence of the steel to be annealed.

2. In a process for annealing columbium or titanium stabilized stainless steel in a protective atmosphere of dissociated ammonia, the step of adding an agent comprising oxygen to the protective atmosphere in amounts sutficient to oxidize the surface of the steel to be annealed and maintain an oxide coating thereon at maximum annealing temperatures and throughout the anneal.

3. In a process for annealing columbium or titanum stabilized stainless steel in a protective atmosphere of dissociated ammonia, the step of adding water vapor to the protective atmosphere to establish the dew point thereof from -55 to -24 F. for annealing temperatures approaching 1750" F.

4. In a process for annealing columbium or titanium stabilized stainless steel in a protective atmosphere of dissociated ammonia, the step of adding water vapor to the protective atmosphere to establish the dew point thereof from 24 to 10 F. for annealing temperatures ranging from 1750 F. to 2050 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,930,388 Hatfield Oct. 10, 1933 2,340,461 Gage et al. Feb. 1, 1944 2,442,223 Uhlig May 25, 1948 OTHER REFERENCES Heat Treatment of Stainless Steel," published by Rustless Iron and Steel Corp., Baltimore, Md. 1944, pages 11-14.

The Iron Age, page 95, February 17, 1949. 

2. IN A PROCESS FOR ANNEALING COLUMBIUM OR TITANIUM STABILIZED STAINLESS STEEL IN A PROTECTIVE ATMOSPHERE OF DISSOCIATED AMMONIA, THE STEP OF ADDING AN AGENT COMPRISING OXYGEN TO THE PROTECTIVE ATMOSPHERE IN AMOUNTS SUFFICIENT TO OXIDIZE THE SURFACE OF THE STEEL TO BE ANNEALED AND MAINTAIN AN OXIDE COATING THEREON AT MAXIMUM ANNEALING TEMPERATURES AND THROUGHOUT THE ANNEAL. 